Thin cat wont gain back weight

Guy is roughly 8 years old and has always been slim in appearance, ranging from 12-14 pounds. His biological brother has always been heftier, usually between 16-18 pounds, and is currently 17 pounds. In the last 6-8 months Guy has been losing a little bit of weight that we thought was from this last winter being so cold and how hard it was to keep our mobile home above 60 degrees. Another of our cats, Leo, who is a bit older at 10 years old, lost a few pounds during that time frame as well. Starting about four months ago I began feeding Guy and the other thinner cat higher quality food morning and night, primarily duck because it has a higher calorie content, and the other cat is back up in weight and is even pushing being chubby. But, Guy hasn't seemed to have gained anything. Each time we've been into the vet in the last three months he's been 10 pounds, so he's not losing anymore, which I think it really good, but he definitely needs to. He's been to the vet four times, twice with our regular vet, twice with a new vet because we weren't getting anywhere with our regular vet. He's had two stool samples, two general blood tests, and two blood tests for thyroid disorders. All of which have come back fine. My regular vets pretty much dismisses me anymore and I'm beyond frustrated with his service. The new vet was just as bad and suggested since the brother cat is so much bigger I'm basing how big cats should be and that Guy is at a good weight because 10 pounds is in the good range of the healthy cat chart she had on the wall. She never even touched Guy and had her assistant do all the handling. We only went back a second time because I hoped the last appointment had been a fluke.

Anyway, now it's been a month since our last vet visit and he's still slimmer than I would like. He eats most of the can he and the other cat split each meal time, which is a variety of duck based flavours from the higher quality brands I can get at petsmart. I also mix a hand full of Fromm's Duck a La Veg into each meal and usually the plate is cleaned off completely within a half hour. The other cat, Leo, usually only eats about 1/3 of the meal and wants out after about 10 minutes, leaving guy in the room with all the rest to eat. The other cats in my home have no access, so I know it's him eating it all. There also have free access to Eukanuba's Indoor hairball relief and another bowl of Eukanuba's sensitive stomach, but Guy very rarely seems to be interested in it anymore. He's just as active, eats a whole lot, no litter box problems, and is all around normal except for the weight loss.

Everyone I've talked to in person seems like I'm over reacting and that he isn't that slim, but I can feel every knob of his back bone and if I pet him without looking sometime I accidentally get my hand under his ribcage. I can feel all the bones in his skull and his tail and his hip. I've never had a cat that wasn't just taken off the street have this much bone sticking out. Even my boyfriend, who has had him since guy was a baby, says he's fine, but I'm really worried about him. Maybe I really am just used to fat cats?

What do you think my next step should be?

If he's eating plenty and still too thin (which it sounds like he is, from your description of how bony he is), the first things that come to mind are intestinal parasites or hyperthyroid. You say those tests all came back negative, but has he been wormed at all? Fecal tests can be very hit and miss, because the worms are not always shedding eggs. If they aren't on the day of the test, you can get a false negative. Also according to my vet there is no test for tapeworm - he could have that without any symptoms other than the weight loss.

A quick google also turned up diabetes as a possibility. Has either of your vets tested for that?

No, I don't believe he has. I will ask my vet about deworming and getting a diabetes tomorrow. Thanks for your reply. I wish my vet would make these suggestions and I didn't have to trawl internet forums for suggestions

I know this was posted forever ago, but I figured I'd reply again. We took him back in and had him wormed. He was given two worming pills, one of which the Dr couldn't get him to swallow before it mostly dissolved. We didn't notice any change in weight and we took him back a few weeks later and they gave him two pills again believing he just didn't get a strong enough dose. He still didn't respond. We've switched his food up some more so he now eats four meals a day that is mixed with a high calorie gel that he absolutely hates the taste of. We trick him into eating it by sprinkling his favorite treats over the top so when he tries to take a treat he gets a mouthful of food as well. And he's gained enough that there is a slight fat layer over his ribs now. I can still feel his backbone and almost every other bones, but he does seem more playful and less grumpy about everything. We had his blood tested again too, which included for diabetes, and all is still fine. He'll be getting a urine analysis next week to make sure the treats and gel aren't too acidic. We've also made him a homemade thunder shirt in the hopes of making him feel more secure thinking maybe stress is causing his weight loss, but so far it's only been for a few days and I'm not sure what to think of it.

Thanks for the update.  Good luck with your cat.  I hope he continues to gain weight.

I'm glad he's gained some weight! I hope he'll continue to improve. It sounds like you're doing everything you can.

We got him to another new vet today and she's tentatively diagnosed him with renal disease. He had new blood work and a combo test and everything was normal again, but she said his kidneys felt small for his body size. She did a urine draw and found blood in his urine. So along with his steady weight loss and the numbers from his urinalysis, she's suspecting renal disease. She gave us a prescription food and an antibiotic for a bladder infection. We'll go back in three weeks for another urinalysis, possibly an ultrasound if she needs more proof, and we'll go from there.